Speed ratio selection in a motor vehicle automatic transmission is conventionally determined as a function of vehicle speed and engine throttle position or torque. When the ratio selection is electronically controlled, the vehicle speed information is derived from a sensor, such as an electromagnetic speed pickup, which generates an electrical speed signal corresponding to the output speed of the transmission.
A potential shortcoming of any such system is that a loss of the vehicle speed signal results in an immediate selection of the first or starting ratio and disables further ratio selection. This shortcoming has been recognized and various approaches have been suggested to minimize the likelihood of such an occurrence. Some systems include circuits for detecting an abrupt loss of the output speed signal; others employ multiple redundant sensors. See, for example, the U.S. Pat. Nos. 4,363,973, Kawata et al. and 4,523,281 Noda et al.
One problem in this regard is that loss of the output speed signal may not be detectable when the vehicle is in a normal idle condition. Indeed, at least one system employs a logical comparison of the output speed signal with various other signals when the vehicle is at rest for the purpose of detecting an inconsistency. See the Stahl U.S. Pat. No. 4,495,457.
However, speed sensor related failures are not the only reason for an apparent loss of the output speed signal. An apparent loss of the output speed signal can also occur when there is a transmission line or operating pressure failure, or merely a failure of the starting ratio of the transmission. In the case of a starting ratio failure, the transmission may be capable of continued operation in a higher ratio; disabling further operation of the transmission, in such case, may result in an unnecessary walk-home situation for the occupants of the vehicle.